[Rcpp-devel] Missing PROTECT()s in forward_exception_to_r()?

[Douglas Bates]

On Mon, Oct 8, 2012 at 6:42 AM, Dirk Eddelbuettel <edd at debian.org> wrote:
>
> Hi Ben,
>
> On 8 October 2012 at 10:24, North, Ben wrote:
> | Hi,
> |
> | I've been using Rcpp for a while now, and finding it very useful ---
> | thanks!
> |
> | Recently, though, I came across strange behaviour when running under
> | gctorture(TRUE) and C++ exceptions were being forwarded to R.  The error
> | message string at the R level was some essentially random pathname, and
> | the string in the C++ exception was nowhere to be seen.  The classname
> | at the R level was the logical TRUE value.
> |
> | The patch below fixed the problem for me.  I would not claim to fully
> | know what I'm doing, so perhaps somebody who does could review and then
> | make the proper fix :-)
>
> It's been a while since we had something like this, and Doug lead it the last
> time. From what I recall, Rf_install() is safe, and the reason we factor
> these out as below. The bad boy, as I call, is probably Rf_mkString so by just doing
>
>  str_what  = Rf_install(exception_what.c_str());
>  str_class = Rf_install(exception_class.c_str());
>  str_ns    = Rf_install("Rcpp"));
>  Rf_eval(Rf_lang3(cppExceptSym, str_what, str_class), str_ns);
>
> may be enough.  Could you test that?

Just glancing quickly at this, I think it may or may not work.
Rf_install looks up a symbol in the symbol table, installing it if
needed, and returns the symbol SEXP.  Rf_mkstring creates a string
SEXP (vector of R class "character" containing a single string).  The
string probably needs protection.  The symbol does not.

I imagine the R function cpp_exception expects strings.  I'm not sure
what happens if it gets symbols.

> |
> | Thanks,
> |
> | Ben.
> |
> | --- src/exceptions.cpp   2012-10-01 01:29:53.000000000 +0100
> | +++ ../Rcpp/src/exceptions.cpp     2012-10-08 11:14:39.435644100 +0100
> | @@ -161,14 +161,23 @@
> |                exception_class = name ; /* just using the mangled name */
> |           }
> |      }
> | -    SEXP cppExceptSym = Rf_install("cpp_exception"); // cannot cause a gc() once in symbol table
> | -    Rf_eval(
> | -        Rf_lang3(
> | -         cppExceptSym,
> | -              Rf_mkString(exception_what.c_str()),
> | -              Rf_mkString(exception_class.c_str())
> | -         ), R_FindNamespace(Rf_mkString("Rcpp"))
> | -    ) ;
> | +
> | +    SEXP cppExceptSym;
> | +    SEXP str_what, str_class, str_ns, ns, expr;
> | +
> | +    PROTECT(cppExceptSym = Rf_install("cpp_exception"));
> | +    PROTECT(str_what = Rf_mkString(exception_what.c_str()));
> | +    PROTECT(str_class = Rf_mkString(exception_class.c_str()));
> | +    PROTECT(str_ns = Rf_mkString("Rcpp"));
> | +
> | +    PROTECT(ns = R_FindNamespace(str_ns));
> | +    PROTECT(expr = Rf_lang3(cppExceptSym, str_what, str_class));
> | +
> | +    Rf_eval(expr, ns); /* Should not return. */
> | +
> | +    // Do this in case somehow someone replaces the definition of
> | +    // "cpp_exception" such that it does return:
> | +    UNPROTECT(6);
> |  }
> |  #else
> |
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> --
> Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com
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